Embodiments of the present invention pertain generally to implantable medical devices, and more particularly pertain to implantable medical devices that automatically calibrate ischemia detection parameters.
Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein. Electrocardiograms (ECG) are useful for diagnosing ischemia and locating damaged areas within the heart. Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. ECGs are composed of various waves and segments that represent the heart depolarizing and repolarizing. ST segment represents the portion of the cardiac signal between ventricular depolarization and ventricular repolarization. While P-waves, R-waves and T-waves may be generally considered features of a surface electrocardiogram (ECG), for convenience and generality, herein the terms R-wave, T-wave and P-wave are also used to refer to the corresponding internal cardiac signal, such as an intra-cardiac electrogram (IEGM).
Techniques have been developed for detecting cardiac ischemia using implanted medical devices. Some conventional IEGM-based ischemia detection techniques seek to detect ischemia by identifying changes in the elevation or depression of the ST segment from the baseline of the IEGM that occur during cardiac ischemia. Elevation or depression of the ST segment in an IEGM may result when there are abnormalities in the polarizations of cardiac tissue during an acute myocardial infraction (MI). An ST segment shift arises because of differences in the electric potential between cells that have become ischemic and those cells that are still receiving normal blood flow. Deviation of the ST segment from a baseline is a result of injury to cardiac muscle, changes in the synchronization of ventricular muscle depolarization, drug or electrolyte influences, or the like.
However, not all ST segment shifts are indicative of MI or other injury to the cardiac muscle. Instead, a ST segment shift above or below the baseline may result because of “axis shifts”, electrical noise, cardiac pacing, high sinus or tachycardia cardiac rates that distort the IEGM waveform. Techniques have been developed for detecting cardiac ischemia using implanted medical devices. However, conventional pacemakers or defibrillators do not monitor disease progression by using a running average of cardiac cycles to determine shifts in the ST segment. Furthermore, typical pacemakers require a user to manually set an isoelectric point, which drifts from patient to patient; thereby preventing any automatic determination of ST shift.
However, conventional approaches to ischemic events have not satisfactorily determined ischemia using ST segment shifts. A need remains for improved methods and systems to detect an acute shift in the ST segment of an intracardiac electrogram, where an ST segment and a fiducial point may be automatically detected and used to determine an ST shift that can be used to assess the existence or exacerbation of a myocardial ischemia.